With or without gene flow? Studying speciation in the Grey-headed Bullfinch species complex

Has there been past gene flow between populations on the Asian mainland and Taiwan?

In 2008, Albert Phillimore and his colleagues concluded that “allopatric speciation is the dominant geographic mode of speciation in birds.” Allopatric speciation refers to the situation where two populations become geographically isolated and genetically diverge, ultimately giving rise to new species. In recent years, however, the focus of speciation research has shifted from a purely geographical perspective (e.g., allopatry, parapatry and sympatry) to the consideration of gene flow. Geographical isolation is still an important component of speciation, but diverging populations often continue to exchange genes during the process. The high incidence of divergence-with-gene-flow examples has led to the idea that this might be the dominant mode of speciation in birds.

Land Bridges

The presence of some gene flow during speciation appears to have become the null hypothesis that many ornithologists start from. For instance, a recent study in the journal Molecular Phylogenetics and Evolution reconstructed the evolutionary history of Grey-headed Bullfinch (Pyrrhula erythaca) species complex. Different populations can be found on the Asian mainland (subspecies erythaca and erythrocephala) and on the island Taiwan (subspecies owstoni). During the Pleistocene ice ages, land bridges connected Taiwan with the mainland, potentially allowing gene flow between these populations. We can thus expect to find some signatures of past gene flow between the Grey-headed Bullfinch populations.

However, genetic analyses clearly separated the mainland and island populations. This separation does not completely rule out past gene flow, so the researchers performed coalescent modelling to test different speciation scenarios. This exercise pointed to a strictly allopatric speciation model. It seems that there has been no gene flow between the Asian mainland and Taiwan.

Sampling locations of the different populations in the Grey-headed Bullfinch species complex. Genetic analyses revealed a clear separation between island and mainland populations. From: Dong et al. (2020) Molecular Phylogenetics and Evolution.

Sky Islands

Why did these populations not exchange DNA despite the land bridges between Taiwan and the Asian mainland? The answer lies in their habitat preferences. Reconstructing the past distributions of these populations revealed that they never overlapped. The mainland populations resided in the mountainous areas and could not expand their ranges to the lowlands. Birds in these “sky islands” were thus isolated from lowland populations, such as the birds expanding from Taiwan. The researchers conclude that “unlike lowland species, incipient sky island species might have had
limited opportunities for intermittent secondary contact and gene flow during late Pleistocene sea-level fluctuations.” Indeed, other highland species, such as the Vinaceous Rosefinch (Carpodacus vinaceus) and the Taiwan Rosefinch (C. formosanus) also diverged without gene flow. The allopatric speciation model is alive and kicking!

References

Dong, F., Li, S. H., Chiu, C. C., Dong, L., Yao, C. T., & Yang, X. J. (2020). Strict allopatric speciation of sky island Pyrrhula erythaca species complex. Molecular Phylogenetics and Evolution153, 106941.

Featured image: Grey-headed Bullfinch (Pyrrhula erythaca) © Robert tdc | Wikimedia Commons

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